The plant architecture of rice includes traits such as tiller number, tillers angle, panicle type and plant height. A good plant architecture is a key factor to improve the yield of rice. A majority of cultivars for an application in production currently are dwarf varieties containing a semi-dwarf gene SD1. The dwarf varieties have many advantages compared with the traditional long-stalk varieties, thus the first “green revolution” was set off and the yield of rice was largely increased. However, a further increase of the yield was limited by the inherent shortcomings of the dwarf varieties, including more unproductive tillers, smaller panicles and higher leaf area index, serious leaf shading phenomenon and decreased canopy photosynthesis and so on. In order to overcome the disadvantage of the limited yield potential of current most cultivars and further meet the people's demand for foodstuff, the breeders of the International Rice Research Institute proposed the concept of new plant type of rice and the main features of the new plant type were fewer tillers, no unproductive tillers, bigger panicles, more grains per panicle, thick and sturdy stems and lodging resistance.
The tiller number of rice is an important agronomic trait in the rice production. The number of productive tillers per unit area decides the number of panicles, which in turn is one of three critical factors to decide the rice yield per unit area. Thus, the reasonable controlling of the occurrence of rice tillering and minimizing the unproductive tillers are of great importance to production.
The grain number per panicle is another important factor in determining the yield of rice. The typical characteristic of most high-yield varieties used currently in production is the significant increase of grain number per panicle. The increase of grain number per panicle is mainly attributed to more primary rachis branch and secondary rachis branch on the panicles and denser grains grown. It is very important to increase the grain number per panicle in breeding high-yield varieties. The 1000-seed weight is the third critical factor to the yield of rice, which is the direct reflection of good or bad dry matter accumulation and grouting in rice grains and is closely associated with the size of the grains.
The lodging resistance ability is always an aspect to which rice breeders have been attaching great importance. It plays a very important role in stabilizing yield and is a limiting factor to further increase the yield, though it does not directly improve the yield of rice. The dwarf varieties enhance the lodging resistance ability through reducing the plant height and thus ensure the stable production of rice and make it possible to increase the yield, compared with the traditional long-stalk varieties. Further enhancing the lodging resistance ability of the plants therefore is critical to further increasing the yield of rice. In the regard, it is always a goal of the breeders to enhance the traits of stem and to breed a variety whose stems are more thick and sturdy and whose lodging resistance ability is stronger.
The basic characteristics of the new plant type proposed by the International Rice Research Institute are few tillers, thick stems and big panicles. The simulation studies suggested that the yield of the variety with new plant type increased by 25% compared with current varieties in the dry season in the tropical regions. It is important to demonstrate the genetic basis and molecular mechanism of tillering, stems and panicles development for obtaining a higher yield variety. Currently, there is no report on genes which may systematically change the plant type of rice overall to produce new plant type characteristics, though many genes associated with the yield were cloned.